A system having an oscillator is used in various kinds of electronic apparatuses, including portable or mobile telephones. In such a system, various operations are performed using an oscillation frequency of the oscillator as a reference, and thus, the oscillation frequency must be maintained constant. However, in actual practice, the oscillation frequency of the oscillator may vary depending on various causes.
The oscillation frequency of the oscillator may vary due to initial deviation, temperature characteristic, power supply fluctuation, aging, and change in environment. Amongst such causes of the oscillation frequency variation, the initial deviation, the temperature characteristic and the power supply fluctuation may be detected prior to forwarding of the electronic apparatus, and such causes may be corrected.
In this specification, the aging refers to a change in the characteristic of the oscillator caused by the aging of constituent elements of the oscillator, and the aging differs for each individual oscillator. In addition, the change in environment refers to a change in the characteristic of the oscillator caused by the change in the environment of the oscillator. For example, the temperature of the environment of the oscillator changes depending on the time of season, and the characteristic of the oscillator also changes depending on the time of season. However, a temperature change depending on the time of season is different for each year.
The aging and the change in the environment are corrected using predicted values. For this reason, in a case where the oscillation frequency of the oscillator shows an unexpected behavior, that could not be predicted by the predicted values, due to the aging or the change in the environment, the oscillation frequency cannot be corrected appropriately. In addition, the aging is caused, for example, by corrosion of an electrode of the oscillator element (crystal) of the oscillator, stress on the electrode at the time of mounting that is released with time, and the like. Further, the change in the environment is caused by the time of season, for example. Accordingly, it is impossible to completely eliminate the change in the oscillation frequency caused by the aging or the change in the environment, by modifying the structural design or the fabrication process of the oscillator.
For example, a Japanese Laid-Open Patent Publication No. 11-55118 proposes a method of writing control data of a programmable oscillator. In addition, a Japanese Laid-Open Patent Publication No. 2000-261334 proposes a method of controlling a voltage controlled oscillator having a variable oscillation frequency based on an external control voltage and a reference clock. In addition, a Japanese Laid-Open Patent Publication No. 2003-264460 proposes a monitoring method that outputs an alarm if an output of the voltage controlled oscillator falls outside a predetermined range.
In the conventional system having the oscillator, there was a problem in that the oscillation frequency of the oscillator cannot be corrected appropriately even if the predicted values is used, in the case where the oscillation frequency of the oscillator shows an unexpected behavior, that could not be predicted by the predicted values, due to the aging or the change in the environment.